Lithium-ion Batteries are Safer Than Ever. That’s not Enough.

Lithium-ion batteries are a powerful technology. While superior energy density is key to its success, it is also associated with risks. As lithium-ion batteries become increasingly integral to our daily lives and the transition to a renewable energy system, manufacturers need to continuously innovate to enhance the safety of lithium-ion batteries and explore alternative chemistries.

Since their commercial introduction, lithium-ion batteries have revolutionized the rechargeable battery market and significantly contributed to the electrification of our societies, powering everything from cellphones and electric cars to telecom base stations.

Lithium-ion batteries are favored for their high energy density, which enhances their performance but also introduces risks, especially thermal events. This downside has been evident in several high-profile incidents all due to fire hazards, including:

• The laptop fires with Dell and Compaq in the early 2000s
• The 2013 grounding of Boeing 787 Dreamliner
• The 2016 recall of Samsung Galaxy Note 7 phones
• The Tesla battery fires in both 2013 and 2018

Unfortunately, incidents do not only happen in consumer products but also in battery energy storage systems. For example, a thermal event occurred in Australia’s biggest battery storage installation, where two of Tesla’s megapacks built on nickel-based cell chemistry (NMC) caught fire in 2021. A similar incident occurred in a Sinexcel BESS container, built on an iron-based cell chemistry (LFP), in the US during the same year.

Despite these challenges, safety advancements have greatly improved, with the failure rates of the most common cylindrical lithium-ion cells decreasing from 1 in 200,000 to 1 in 10 million over the past two decades.

Types of Battery Failures and Mitigation Strategies

Battery failures can be categorized into benign and hazardous types. Benign failures, such as capacity fade, result in reduced or complete loss of function. Hazardous failures, such as thermal runaway, create immediately dangerous situations by triggering violent chemical reactions that produce excessive heat. If not properly managed, for example by controlling or venting hot gases, thermal runaway can lead to intense fires or explosions. This becomes even more critical if the thermal runaway propagates to neighboring cells, causing cascading failures. Common causes of thermal runaway include:

• Over-charging
• Over-discharging
• High temperatures
• Low temperatures
• Mechanical damage
• Poor cell design
• Manufacturing defects

The Role of Battery Management Systems in Ensuring Safety and Reliability

To mitigate these failures, batteries are designed with multiple layers of safety, from the cell level up to the battery system level. Cell level safety is achieved by using current interrupt devices that deactivate the cells if pressure becomes excessive. In a module, the battery can be equipped with insulating walls or sheets to prevent or slow down fire propagation, and the module itself can protect the cells from mechanical damage. On system or rack level fire suppression systems control and quench hazardous situations. Polarium’s in-house developed battery management system (BMS) can be placed in both the module, the pack, or the rack depending on system size. The BMS provides a means of monitoring the cell parameters to ensure that the battery is always operating within its safe operating limits, as well as a means of electrically disconnecting the cells from the outside electrical system if these safety limits are exceeded.

Although the BMS will always protect the battery, the battery should not be operated in such a way that the BMS is forced to disconnect it for safety reasons. The power system (PCS), which charges and discharges a battery, should be integrated with the BMS and include means to reduce power before the safety systems engage. The BMS also gathers operational data to enhance intelligent features and add-ons, resulting in adaptable and sophisticated battery systems. Altogether, these rigorous safety measures not only avert potential thermal events but also optimize the charging and usage of batteries, significantly extending the batteries’ lifespan and reliability.

Furthermore, the selection of cells will also impact safety. LFP is perceived to be safer than NMC, which is true to some extent. The onset temperature for a thermal event is slightly higher for LFP, so the margins can be better. However, the most critical aspect of a battery’s safety is its possibility to handle a thermal event. Cell size is of great importance in this regard, as the energy released in case of a thermal event is directly related to the cell capacity. LFP cells are many times larger compared to NMC cells, and a thermal event in large cells will be harder to contain and prevent from cascading.

Failure Tests

Safety certification has evolved together with the industry to ensure that all products entering the market adhere to safety requirements protecting consumers. During certification, a battery is exposed to several abuse conditions that aim to simulate events that may occur during the battery’s lifecycle, to ensure that it still provides a minimum safety level defined by the standard.  There are regional differences in terms of standards (for example EN in Europe, UL in North America, and GB in China), but the certification tests are similar. The safety tests include:

• External and internal short-circuits
• Over-charge
• Over-discharge
• Propagation (single cell failure)
• Over-temperature
• Drop tests
• Impact and crush tests
• Vibration and shock tests
• Fire exposure tests
• Low-pressure tests

Shaping the Future of Battery Safety

Introducing a cell-agnostic approach, Polarium embraces flexibility in seamlessly integrating diverse cell types and chemistries. This approach ensures that our systems lead in adaptability and innovation. One promising advancement is sodium-ion battery technology which exhibits excellent performance in terms of longer life, more flexible working temperatures, and safety. However, their energy density currently falls short of lithium-ion batteries, limiting their suitability for certain applications. Ongoing research and development efforts are set to enhance sodium-ion batteries’ commercial readiness.

Nonetheless, significant advancements in product development and evolving regulatory standards have contributed to a considerable risk reduction associated with lithium-ion batteries. As lithium-ion batteries are a core technology in the transition to a sustainable future, they will become an even more integral part of our daily lives going forward.

For example, the International Energy Association estimates that almost half of the cars sold globally in 2035 are expected to be electric, compared to 18 percent in 2023.[1] In addition, lithium batteries are also a key enabler in the transition to a renewable energy system, as they enable intermittent renewable energy to be stored and turned into a dispatchable asset. According to the World Economic Forum, the global demand for lithium-powered energy storage is expected to fourfold between 2022 and 2030.[2]

This rightfully sets a high bar for battery safety standards for everyone involved in the lithium-ion battery value chain. Even though the industry has come a long way in reducing safety risks, we can never sit back. As the batteries continue to evolve, so do the risks. Battery testing and safety are as important now as it was a few decades ago. If not more so.

By: Ulf Krohn

Citations

• [1] International Energy Agency, Global EV Outlook 2024
• [2] Ian Shine, World Economic Forum, The world needs 2 billion electric vehicles to get to net zero. But is there enough lithium to make all the batteries?